Can you give advice for a 14 year old? He has a reading comprehension of a 3rd grader, and it is not improving. He is homeschooled, and I would love to know if there is any curriculum at his level, but for 8th grade subjects. Several have been used, but would love for him to experience more science, reading, and history.

Dave,
I would start with lots of books on CD. Even if his reading comprehension is low, he should do better with listening to the stories. Even my 4 yr old loves to hear The Chronicles of Narnia, The Hobbit, The Call of the Wild, White Fang, and many other stories. You can get historical books on CD too. Choose books that would be at a middle school level or higher.
For science, do lots of experiments. Even if he doesn't read well, if he sees something, he can get it. There are lots of great science curriculums out there and lots of recommendations on this site. I personally use the Supercharged Science e-science curriculum, so you can check that out if you want at www.SuperchargedScience.com/esci.htm.
I hope this helps.